SANAA, YEMEN - Security forces used water cannons, tear gas and live ammunition to quell anti-government activists in the capital early Saturday, witnesses said, but were forced to retreat after hours-long clashes in which at least two people were reportedly killed and hundreds were injured.

Pressure had been building at the opposition's tent camp at Sanaa University since Friday evening, when activists reported that security forces and armed pro-government thugs were gathering around the barricaded site. Shortly after the dawn prayer, the security forces deployed a water cannon against demonstrators, according to eyewitness reports.

A battle broke out at the edge of the camp in which protesters used pickaxes to break the paved sidewalk and smash it into baseball-size chunks that they threw at the security forces.

Ibrahim Babulraik, 16, a student, said that security forces fired poison gas and bullets at him and other demonstrators, but he appeared undeterred. "I will come back tomorrow," he said, "I will stay until we become martyrs."

Abdulaleem al-Subriy, a physician at the hospital, confirmed that a man had died from gunshot wounds to the head after the early-morning clash. There were also reports of at least one other death in a separate incident at the camp.

The Interior Ministry issued a statement denying that live ammunition had been used against demonstrators, saying that the shots were fired by "third-party instigators." It said local residents had threatened to resort to violence if police did not restrain the protesters on their doorsteps and added that security forces had been forced to intervene to prevent clashes between demonstrators and residents several times Friday afternoon.

The demonstrators set up camp outside one of the gates of Sanaa University three weeks ago and have been expanding their territory since then. They now hold a district around the university, which they police themselves. Concrete roadblocks and stone barricades prevent cars from entering, and security teams check bags and frisk for weapons at all the entrances.

Hundreds of activists are sleeping in the camp and say they will stay until Yemen's president of almost 33 years, Ali Abdullah Saleh, steps down. "He doesn't have a choice," said one of them, 23-year-old student, Mohammed Hameddien. "He has to go."

Earlier Saturday, 12 anti-government demonstrators were injured when security forces opened fire in the city of Taiz, according to the Associated Press.

Yemen's president is a crucial ally in U.S. efforts to combat the al-Qaeda cell active in the country. In 2010, the Saleh government received $300 million in aid from the United States.

In a telephone call Friday with Saleh, U.S. counterterrorism chief John Brennan urged both parties to find a peaceful solution to the ongoing crisis. "Brennan reiterated that representatives of all sectors of the Yemeni opposition should respond constructively to President Saleh's call to engage in a serious dialogue to end the current impasse," according to a White House statement.

The ongoing wave of unrest in Yemen has led to more than 30 deaths in recent weeks, according to international human rights groups. Last week, U.S. citizens and other foreign nationals were advised to leave the country.

In Bahrain, meanwhile, tens of thousands of protesters encircled one of the royal family's palaces in the capital, Manama, shouting calls for political freedom and the king's ouster, AP reported. The demonstration came a day after a similar march triggered a violent response from security forces, but there was no repeat of the violence Saturday.

In Jordan, thousands of people took to the streets of Amman, the capital, to voice allegiance to King Abdullah II in the biggest demonstration there in weeks. AP reported that the turnout was three times as large as for recent opposition protests demanding a wider public say in politics, more jobs and lower food prices.